I exchanged 20% of my Bitcoins to Litecoins recently because I think they have the most potential to be #2 of the cryptocoins. At the moment Litecoins are more established than all the other ones. There is even a SR alternative called Atlantis where goods are traded for LTC since last week, which could help to increase their value and use.

This may change in the futurre but right now I think the answer to your question is: Litecoins